It is useful for products powered by batteries, energy-harvesting systems (e.g., solar cells or thermoelectric scavenging), or other variable sources to be able to monitor the voltage supplied by the source. For example, a useful feature for battery-powered products is a “low-battery alert,” an alert to notify the user in time to replace the battery before it is completely discharged and service is interrupted.
In addition, it is useful to detect conditions when the power source is unable to support the load presented by the product; upon detection of such a condition the product may be designed to disconnect non-essential loads to preserve the function of essential loads (“load shedding”) or take other compensatory action to extend its operational life. Existing products frequently perform this detection by monitoring the voltage supplied to the product by the power source.
Since the internal impedance of the power source may not be negligible, the voltage supplied to the product may vary under varying load conditions. To ensure that an undetected under-voltage condition does not occur, substantially-constant monitoring of the voltage supplied by the source is therefore required. Existing insufficient-supply detectors typically require a dedicated analog comparator and the generation of a reference voltage to use in the comparison process. These circuits consume additional power, and so in many products are often rarely enabled. Undetected insufficient-supply conditions, therefore, can occur, leading to undesired product behavior.
Since battery-powered products frequently are portable or mobile, it is desirable that their insufficient-supply detectors have small physical size and low power consumption. To achieve this, it would be helpful if the detection can be derived from already available information. It would also be helpful (for power minimization and cost minimization) if the detection circuit is digital.
What is needed is an insufficient-supply detector that is consistent with these requirements.